The electronic display is used in such devices as television sets, computer terminals, telecommunications equipment and a host of other applications as well. No other communication medium offers its speed, versatility and interactivity. Among the types of electronic displays currently available, there is no doubt that the technology concerning flat panel displays is of a significant interest and progress is continuously being made in this field. For example, according to S. W. Depp and W. E. Howard, ("Flat Panel Displays", Scientific American 90-97 (March 1993)), incorporated herein by reference, flat panel displays were expected to form a market of between 4 and 5 billion dollars in 1995 alone. Desirable factors for any display technology include the ability to provide a high resolution, full color display at good light level and at competitive pricing.
Organic light emitting devices (OLED's), which make use of thin film materials which emit light when excited by electric current, are becoming an increasingly popular form of flat panel display technology. Presently, the most favored organic emissive structure is referred to as the double heterostructure (DH) OLED, shown in FIG. 1A. In this device, a substrate layer of glass 10 is coated by a thin layer of indium-tin-oxide (ITO) 11. Next, a thin (100-500 .ANG.) organic hole transporting layer (HTL) 12 is deposited on ITO layer 11. Deposited on the surface of HTL 12 is a thin (typically, 50 .ANG.-500 .ANG.) emission layer (EL) 13. The EL 13 provides the recombination site for electrons injected from a 100-500 .ANG. thick electron transporting layer 14 (ETL) with holes from the HTL 12. Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870, the disclosure of which is incorporated herein by reference.
Often, the EL 13 is doped with a highly fluorescent dye to tune color and increase the electroluminescent efficiency of the OLED. The device as shown in FIG. 1A is completed by depositing metal contacts 15, 16 and top electrode 17. Contacts 15 and 16 are typically fabricated from indium or Ti/Pt/Au. Electrode 17 is often a dual layer structure consisting of an alloy such as Mg/Ag 17' directly contacting the organic ETL 14, and a thick, high work function metal layer 17" such as gold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17" is opaque. When proper bias voltage is applied between top electrode 17 and contacts 15 and 16, light emission occurs from emissive layer 13 through the glass substrate 10. An LED device of FIG. 1A typically has luminescent external quantum efficiencies of from 0.05% to 2% depending on the color of emission and the device structure.
Another known organic emissive structure is referred to as a single heterostructure (SH) OLED, as shown in FIG. 1B. The difference between this structure and the DH structure is that multifunctional layer 13' serves as both EL and ETL. One limitation of the device of FIG. 1B is that the multifunctional layer 13' must have good electron transport capability. Otherwise, separate EL and ETL layers should be included as shown for the device of FIG. 1A.
Yet another known LED device is shown in FIG. 1C, illustrating a typical cross sectional view of a single layer (polymer) OLED. As shown, the device includes a glass substrate 1 coated by a thin ITO layer 3. A thin organic layer 5 of spin-coated polymer, for example, is formed over ITO layer 3, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer 6 is formed over organic layer 5. The metal is typically Mg, Ca, or other conventionally used low work function metal.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in U.S. Pat. No. 5,294,870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light. Fluorescent media are positioned between the blue OLED and the substrate in certain parts of the pixels. The fluorescent media absorb light emitted by the blue OLED and emit red and green light in different regions of the same pixel. One drawback of this display is that waveguiding of light through the glass substrate from one pixel to adjacent pixels of different color can result in blurring, color bleeding, lack of image resolution and the loss of waveguided light. This problem is schematically shown in FIG. 1D for a device shown in FIG. 1A, and is further described in D. Z. Garbuzov et al., "Photoluminescence Efficiency and Absorption of Aluminum Tri-Quinolate (Alq.sub.3) Thin Films," 249 Chemical Physics Letters 433 (1996), incorporated herein by reference. A further problem in this device is that the ITO used as a transparent, conductive layer is a high-loss material, thus resulting in absorption of waveguided light by ITO layers. One additional problem encountered in this and other prior art devices is that the LED interconnect lines can be seen by this viewer as black lines surrounding individual pixels, thus increasing the granularity of the display and limiting resolution.